Metering devices are used extensively in the health care industry. Typical of the applications are syringe pumps for delivering medicaments directly into the patient's bloodstream, wherein a stepper motor drives a lead screw to move the plunger of the syringe to dispense the medicament. The metering is accomplished by controlling the motor speed and hence the frequency of plunger travel. These syringe pumps suffer a number of drawbacks; they are relatively expensive; the dosage accuracy depends on the diameter of the plunger, which may vary with different manufacturers; particularly at low flow rates the plunger can suffer from stiction, which causes variations in the delivery rate; and a drive signal to the stepper motor does not always result in movement of the motor shaft if there is significant resistance to flow. On the positive side, they are easy to set up and maintain, fairly reliable, and generally convenient to use.
Another type of metering pump used extensively in hospitals and for ambulatory drug delivery devices is the peristaltic pump. This consists of an elastomeric tube containing a fluid, one end of the tube being connected to a reservoir of the fluid to be pumped, and at least one pair of rollers spaced apart which are pressed onto the tube to trap a bolus of fluid therein, and which traverse along the tube to move the bolus towards an outlet. Such pumps employ a number of rollers around a drum or on a linear track, so that there is a slightly intermittent delivery rate of fluid as each bolus is delivered. The intake suction is created by the tube springing out after the passage of a roller. Peristaltic pumps deliver a reasonably accurate flow when the pump tubes are new, but the tube material quickly loses some of its elasticity and the flow rate drops. Furthermore, it is quite common for tubes to split unless changed frequently. The good features are that they are simple to set up and maintain; no glands or sliding seals are required; the tubes are often contained conveniently within a disposable cartridge, including the rollers; they are relatively inexpensive.
Another metering device used very commonly in hospitals is the very simple drip controller which is used in conjunction with intravenous infusion sets. The basic device consists of a clamp which is adjusted to partially occlude a resilient tube leading from the infusion drug container. The flow rate is determined by counting the drops within a viewing chamber; some equipment monitors the drip rate electronically. They are less complicated than pumps, and the fluid is pressurised by fixing the container above the delivery level. They have no moving parts, and few maintenance problems. However, because of changes in the pressure head as the infusion drug is consumed, the flow rate falls, and such controllers require frequent adjustment. Although various types of tube clamps are in use, inevitably the soft tubing material creeps after the initial setting, necessitating further adjustment. Also they present difficulties when used with viscous solutions. More sophisticated controllers use feedback to control the variable restrictor, but there are few, if any, suitable flow measuring devices that can cover the range of flow rates required with accuracy, at a reasonable cost.
With the first two examples, the pump could build up a pressure on the delivery side if a blockage occurs, and a pressure warning device is essential. In the last example of a metering device, a blockage downstream would merely cause the flow to cease, but again, an alarm is essential.
Another metering device used for drugs is the metered dose inhaler (MDI) which is used to deliver successive doses of drugs in a spray of fine droplets having a mean diameter of about 5 microns. Generally the device contains a drug mixed with a liquefied gas propellant; a metered volume of the mixture is first isolated from the bulk, and then opened to atmosphere, whereupon the propellant boils almost explosively and dissipates the drug as a fine spray. Until recently such MDI's relied entirely on the use of CFC's as the propellant, but with the alleged ozone depletion caused by such chemicals, new ways have been found to power MDI's, such as the device co-invented by the present inventor, described in WO 91/14468, which employs very high pressure to atomise the metered dose of drug. However, this device is bulky compared to propellant MDI's, since it employs a spring-loaded piston which must be compressed, latched, and released on demand to dispense the dose.
In addition to the clamp type controller discussed, other types are needle valves, which are viscosity sensitive and suffer from particulate contamination which increases flow resistance, and flow interrupts such variable frequency on-off valves, which are very viscosity sensitive.
Hence it may be seen that there are two basic ways of metering medical fluids: by flow rate control of pressurized fluids, as in the tubing clamp on IV infusion sets, and by positive displacement pumps, such as syringe pumps. The former are usually inexpensive and inaccurate, the latter just the opposite. Generally it is simple and inexpensive to pressurize fluids, for example by storing them in a container with compressed gas, or by placing a weight onto a closed bag of fluid. However, to the present inventor's knowledge, none of the flow control elements currently available have sufficient accuracy and/or stability to give accurate dispensing over a wide range of delivery rates.
These problems may be overcome by introducing feedback, so that the delivered flow rate is compared with a set value, and the controlling element adjusted accordingly. The resulting devices are more complex, larger, expensive, and generally unsuitable for small flow rates. For many applications, the fundamental problem is that there are no accurate flow measuring devices which are capable of a wide dynamic range, suitable for a wide range of viscosities, and costing very little to manufacture. Among methods in use today are impellers which are placed in the fluid stream and rotate according to the flow rate; Doppler effect devices; and thermal, ultrasonic, optical, and gravimetric instruments. Most have serious drawbacks, such as sensitivity to viscosity, density, change from laminar to turbulent flow, a variable velocity profile within the measuring conduit, and environmental change. Tremendous advances in metrology science have enabled all of these problems to be solved for particular applications, but always at the expense of limited range or difficult fluid conditioning requirements such as ultra filtration.
It is recognised that there never will be one solution for the infinite variety of fluid metering requirements, but nevertheless there is still scope for significant advance in the art.